Under laboratory conditions, light from a source of light being analyzed can be precisely aligned with a spectrograph instrument and passed through a slit. Light from a source having known spectral lines can also be aligned with the instrument and passed through a separate portion of the slit to provide spectral reference lines. However, some light sources that are desired to be analyzed by spectroscopy are short in duration, or are moving, or occur in groups of point light sources, so that precise aiming of the spectrograph is not possible.
To obtain the spectra of such light phenomena, slitless spectrographs have been designed. Since it is impossible to use a precisely aimed known light source for calibration, various methods have been proposed for providing spectral reference marks on the spectrogram recorded by a slitless spectrograph. In connection with the use of spectrographs to determine the radial velocities of stars, methods have been proposed for recording interference bands, for recording absorption lines by placing an absorbing medium in the optical path, and for recording spectral lengths from a recognizable stellar spectral line. A widely used method is the reversal method in which the spectrum of a star is simultaneously photographed next to a reversed spectrum of the same star.
I. D. Liu has described a slitless spectrograph, in which measurements of the undispersed or "zero order" light from the source is recorded on the same film frame as the first or higher order spectral lines, in an article entitled "Spectra Associated With High-Speed Projectiles In Rarified Air Obtained With Large-Aperture Slitless Spectrograph", in APPLIED OPTICS, September, 1962, Volume 1, No. 5. Liu notes that the zero order mark can be used to calibrate the remainder of the spectrum. However, this method has several disadvantages arising from the fact that the zero order mark is recorded at its normal distance from the first order spectrum. This distance is proportionately large, and a small scale is required to encompass both the zero and later order spectra, resulting in less accuracy in measuring the spectrogram. Also, this method provides only one reference mark. Therefore, the wave lengths of the unknown lines must be calculated by extrapolation, which is not as accurate as interpolation between two known reference marks.